I. Field of the Invention
The present invention relates generally to the manufacture of hollow turbine blades and, more particularly, to an improved method of manufacturing to readily and accurately obtain complex hollow passage geometry required for modern turbine blades.
II. Description of the Prior Art
Modern turbomachinery utilizes airfoils which are variously referred to as vanes or blades. Blades, sometimes called "buckets", rotate and are used in disk assemblies of one type or another. Vanes, sometimes called "stators", do not rotate and are found in stationary parts such as nozzles, cases, diffusers, and similar types of turbine engine parts.
Blades, in turn, can be referred to as integral or separate. Integral blades are bonded to and/or part of a disk (sometimes called a "blisk") or ring (sometimes called a "bling") assembly. In these cases, there is not real root area to consider unless after bonding a noticeable material or structural dissimilarity still exists between the original disk or ring and the original blade. When the original blade and the original disk or ring are produced together (integrally) there is no noticeable difference in structure and therefore never a reference to "roots". Separate blades are produced having precision machined roots which are assembled into machined disks.
Vanes also come in two varieties: separate or integral. In this instance, "integral" means cast at the same time as the nozzle's inner or outer shrouds. Separate vanes are cast as individual vanes and then either fastened, brazed or bicast into the inner and outer shrouds.
Because of the high operating temperatures of most turbines, many turbine blades as well as turbine vanes require air cooling to keep their operating temperature below the melting point. As engines become more and more demanding on their hardware, the cooling schemes employed internal to the airfoils increase in complexity. It becomes more difficult to achieve the desired cooling passage configuration and its complexity, particularly in the very small airfoils envisioned for the future.
Cooling passages are desirably central to the airfoil. That is, they are "balanced" in the cross section of the airfoil so that the wall thicknesses on either side of the passage are neither too thin or too thick. Wall thickness does not have to be uniform throughout and may vary either intentionally to locally control cooling effectiveness or unintentionally due to normal process variation. Cooling passages may be serpentine like with parallel or near parallel passages running lengthwise through the airfoil connected by near 180.degree. bends at either end. There may occasionally be cross overs between the passages. Usually the cooling passage has one end which exits the airfoil at its root. The other end exits through either the top or the trailing edge of the airfoil. Some passages "dead end" in the airfoil. That is, the passage has only one exit to the outside. These are called "blind" passages. Cooling air entering the passage effuses out of the blind passage through minute holes drilled through the airfoil walls into this passage. Passages are formed within an airfoil (either blades or vanes) during the investment casting process by means of a ceramic core which exhibits the exact shape of the hollow passage geometry being sought. With respect to the surrounding metal in the airfoil, the core is like a negative. For example, if it is desired that a metal post project into a cooling passage, this positive feature (that is, the post) would require a corresponding negative feature in the core (that is, a hole).
Heretofore, it has been customary to use machined tools and dies to inject core material into die cavities in order to obtain the features desired. Features whose smallest dimension is greater than approximately 0.020 inches can usually be produced today using conventional core technology. However, for features dimensioned smaller than that value, conventional core technology is inadequate. For features dimensioned as small as approximately 0.005 inches, the laser has been found to be an excellent tool.
Laser machining, however, does have a history. For example, U.S. Pat. No. 4,606,747 to Steinhoff and U.S. Pat. No. 4,401,876 to Cooper both disclose contact-less removal of material from the surface of an article of brittle material by means of a laser beam. U.S. Pat. No. 4,978,830 to Millerick et al. discloses automatic apparatus for laser trimming semiconductor integrated chip packages. U.S. Pat. No. 4,986,664 to Lovoi, U.S. Pat. No. 4,952,789 to Suttie, and U.S. Pat. No. 4,914,270 to Copley et al. all disclose methods and apparatus for the controlled laser removal of material from a substrate. U.S. Pat. No. 4,970,600 to Garnier et al. and U.S. Pat. No. 4,467,172 to Ehrenwald et al. both disclose methods and apparatus for engraving a workpiece by means of a laser. U.S. Pat. No. 4,475,027 to Pressley discloses apparatus comprising a laser and optical beam homogenizer which is useful for metal hardening, semiconductor hardening, and other materials processing applications. U.S. Pat. No. 4,322,601 to Serlin discloses a method and apparatus for alloying the surface of a substrate by use of a laser beam.
It was in light of the prior art, typical examples of which have just been described that the present invention was conceived and has now been reduced to practice.